buzz in amp goes away only when amp is dimed

[blindopher]

Hi,


I'm trying to trouble shoot a buzz that has recently developed in my amp after a year of being very quiet.

The buzz gets louder as the volume hits 12 oclock, then starts fading and disappears just after 3 oclock when amp is basically dimed.

The amp in question is the Mod 102 amp kit. A 5W SE non master volume amp with a 12AX7 and EL84.

The link below contains all info and schematics.


http://www.modkitsdiy.com/kit/mod_102_guitar_amp_kit


nothing else besides the volume control has any effect on the buzz.

The amp is buzzing with nothing plugged in or with an input signal - same buzz. It's not the guitar.


The buzz is omnipresent at all locations.

It is definitely a buzz and not a hum. It's not the tubes either - i swapped them with the same results. I'm quite a noob having only 2 amp projects under my belt, but that volume phenomena is very suspicious. Any clues and educated guesses would be greatly appreciated before i go inside the chassis.


thanks!

[casssax]

I would check the coupling cap. There could be DC voltage leaking into the volume pot.

I had a leaky coupling cap and it had similar effects.

The volume would get louder to up to a point and then start getting quieter and there was a buzz as the volume got louder.

[Willabe]

I would check the coupling cap. There could be DC voltage leaking into the volume pot.

Maybe you already know this (we don't know how experienced you are), so with the amp off, set up your meter for DCV, then clip the meter's + to the volume pot's input leg end, where the coupling cap (CC) comes in/feeds and the - to ground.

Then turn the amp on and take DCV readings at different turns/rotations on the volume pot.

Post your readings.     (Should be less than 0.5dcv if the CC isn't leaking dcv.)

 

[blindopher]

I'm probably missing something here, but looking at the schematics i see my volume control is NOT fed by a coupling cap. The signal comes off the V1A plate into a Fenderish looking TB tonestack and volume, then fed directly to the grid of V1B, and only then as it comes off the V1B plate does it see a cap before the V2 (EL 84 output tube).


Are you refering to the tonestack caps? Do they double as blocking caps for the V1A stage? If so there are three - which one should i check?


Or are you refering to the .022u cap before the output tube?


thanx!

[Willabe]

Yes in your amp the TS caps are being used as the coupling cap.

Measure for DCV at the top of the volume pot or the treble pot wiper.

[sluckey]

I would also examine all ground connections.

[blindopher]

Thanks everyone, and this is what i found out:


1. I ran continuity on all chassis grounds and all is good.


2. BUT, when i started checking voltages (to test for leaky coupling cap) at the top of my volume pot and turned the volume up i immediately got a LOUD hum very much like touching the tip of your guitar cable, only MUCH louder. (The above scenario is when my DMM is grounded to chassis and i'm probing the pot with my red). Needless to say i did not continue. Is this normal and i was just foolish not to have used a dummy load?


3. Worst of all, i suddenly discovered that by touching the volume knob and pressing hard over the screw which fastens it to the pot shaft i'm affecting the buzz! Now it seems like a grounding problem and i can only think of the volume pot as perhaps being defective, especially since at the extreme rotation (when amp is dimed) the buzz suddenly goes dead quiet and all is good. (I don't mean gone because of signal to noise ratio - i mean real gone.)


Any advise?

[birt]

well if you have a spare pot around just disconnect the original one without removing it and test with the spare. that should get you started. even if the spare is a different value, it will be obvious if a good functioning pot solves the problem.